Over the last two decades, integrated whispering-gallery-mode resonators have been increasingly used as the basic building blocks for selective filters, high-sensitivity sensors, and as nonlinear converters. In the latter two cases, optimum performance is achieved when the intra-cavity power or the resonance feature contrast are maximum. For devices with transversely singlemode resonator and access waveguides, the above-mentioned conditions are obtained when the system is critically coupled i.e. when the coupler power transfer rate corresponds to the single-pass intra-cavity loss. Designing coupled resonators for which critical-coupling is maintained over a large spectral range is therefore attractive to facilitate sensing or nonlinear frequency conversion.

In this paper, we theoretically show, using a generic model based on the universal description of the device spectral characteristics and a coupled-mode theory treatment of the coupling section, that access-waveguide-coupled resonators can exhibit a wideband critical-coupling bandwidth when their constitutive resonator and access waveguides are different i.e. when they are phase-mismatched. To illustrate this, we have calculated the spectral response of Si₃N₄/SiO₂ racetrack resonators and have found that, when the coupler beat-length becomes achromatic, the device critical-coupling bandwidth is expanded by more one order of magnitude compared to their phase-matched counterpart.

Date Published: 16 February 2018
PDF: 7 pages
Proc. SPIE 10518, Laser Resonators, Microresonators, and Beam Control XX, 105180I (16 February 2018);

Published in SPIE Proceedings Vol. 10518:
Laser Resonators, Microresonators, and Beam Control XX
Alexis V. Kudryashov; Alan H. Paxton; Vladimir S. Ilchenko, Editor(s)